1. Field
Exemplary embodiments relate to a method and apparatus for downmixing multi-channel audio signals.
2. Description of the Related Art
Due to development of multimedia processing techniques, various audio channels are available. Compared to single-channel (mono) audio signals and 2-channel (stereo) audio signals, 5.1-channel audio signals and 7.1-channel audio signals are commonly used, and audio devices capable of outputting even more audio channels are being manufactured.
To perfectly output such multi-channel audio signals, audio devices supporting multi-channel audio signals are required. Therefore, mobile devices with limited available power, limited signal processing resources, and a limited number of output speakers are unable to properly output multi-channel audio signals. Therefore, mobile devices encode multi-channel audio signals into stereo-channel audio signals or mono-channel audio signals. The encoding is referred to as downmixing.
FIG. 1 is a block diagram for describing a common process for downmixing multi-channel audio signals.
As shown in FIG. 1, bitstreams of multi-channel audio signals are output to block 110 and unpacked therein. In block 120, unpacked data is inversely quantized and frequency coefficients are respectively restored with respect to multi-channels.
In block 130, each of the multi-channel frequency coefficients is converted into a signal in the time domain via an inverse transform. For example, in a case of downmixing a 5.1 channel bitstream to a stereo-channel bitstream, In the block 130, an inverse transform is performed on each of the 5 channel frequency coefficients in the block, and thus 5 frequency coefficients are generated. Generally, in a case of downmixing 5.1 channel audio signals, signals in a low frequency effects (LFE) channel are discarded. Here, the inverse transform is a process for converting signals in the frequency domain into signals in the time domain, where an inverse fast Fourier transform (IFFT) is generally employed.
In block 140, levels of audio signals in the time domain converted from the multi-channel frequency coefficients are suitably adjusted for channels, and the adjusted multi-channel audio signals are downmixed to stereo-channel audio signals. Generally, levels of 5.1 channel audio signals are adjusted while the 5.1 channel audio signals are being downmixed to stereo-channel audio signals.Lo=L+0.707C+0.707Ls Ro=R+0.707C+0.707Rs 
(Lo, Ro: Stereo Left/Right, L: left, R: Right, Ls: Left Surround, Rs: Right Surround, C: Center)
In block 150, post-processing required by an audio codec (e.g., overlap and add process) is performed and final stereo-channel audio signals are output.
In such a common downmixing method, the number of channels in source audio signals may be reduced, and thus multi-channel audio signals may be converted into stereo-channel audio signals suitable for mobile devices. However, such a downmixing process requires a large amount of power and resources. Particularly, the inverse transform process involves a large amount of calculations. Here, since the power and resources consumed increase as the number of channels of audio signal source increases, a method of downmixing multi-channel audio signals requiring relatively fewer calculations and less power is necessary for devices with limited performances, such as mobile devices.